Misc Topics
Amol Deshpande
CMSC424
Topics
 Today
 Database system architectures
 Client-server
 Parallel and Distributed Systems
 Object Oriented, Object Relational
 XML
 Next class…
 Data warehouses, Information Retrieval, Database Tuning ?
Database System Architectures
 Centralized single-user
 Client-Server Architectures
 Connected over a network typically
 Back-end: manages the database
 Front-end(s): Forms, report-writes, sqlplus
 How they talk to each other ?
 ODBC:
– Interface standard for talking to the server in C
 JDBC:
– In Java
 Transaction servers vs. data servers
Database System Architectures
Parallel Databases
 Why ?
 More transactions per second, or less time per query
 Throughput vs. Response Time
 Speedup vs. Scaleup
 Database operations are embarrassingly parallel
 E.g. Consider a join between R and S on R.b = S.b
 But, perfect speedup doesn’t happen
 Start-up costs
 Interference
 Skew
Parallel Databases
 Shared-nothing vs. shared-memory vs. shared-disk
Parallel Databases
Shared Memory
Shared Disk
Disk interconnect
is very fast
Shared Nothing
Communication
between
processors
Extremely fast
Scalability ?
Not beyond 32 or Not very scalable Very very
64 or so (memory (disk interconnect scalable
bus is the
is the bottleneck)
bottleneck)
Notes
Cache-coherency Transactions
an issue
complicated;
natural faulttolerance.
Distributed
transactions are
complicated
(deadlock
detection etc);
Main use
Low degrees of
parallelism
Everywhere
Not used very
often
Over a LAN, so
slowest
Distributed Systems
 Over a wide area network
 Typically not done for performance reasons
 For that, use a parallel system
 Done because of necessity
 Imagine a large corporation with offices all over the world
 Also, for redundancy and for disaster recovery reasons
 Lot of headaches
 Especially if trying to execute transactions that involve data from multiple sites
 Keeping the databases in sync
– 2-phase commit for transactions uniformly hated
 Autonomy issues
– Even within an organization, people tend to be protective of their
unit/department
 Locks/Deadlock management
 Works better for query processing
 Since we are only reading the data
Next…
 Object oriented, Object relational, XML
Motivation
 Relational model:
 Clean and simple
 Great for much enterprise data
 But lot of applications where not sufficiently rich
 Multimedia, CAD, for storing set data etc
 Object-oriented models in programming languages
 Complicated, but very useful
 Smalltalk, C++, now Java
 Allow
 Complex data types
 Inheritance
 Encapsulation
 People wanted to manage objects in databases.
History
 In the 1980’s and 90’s, DB researchers recognized benefits of
objects.
 Two research thrusts:
 OODBMS: extend C++ with transactionally persistent objects
 Niche Market
 CAD etc
 ORDBMS: extend Relational DBs with object features
 Much more common
 Efficiency + Extensibility
 SQL:99 support
 Postgres – First ORDBMS
 Berkeley research project
 Became Illustra, became Informix, bought by IBM
Example
 Create User Defined Types (UDT)
CREATE TYPE BarType AS (
name CHAR(20),
addr CHAR(20)
);
CREATE TYPE BeerType AS (
name CHAR(20),
manf CHAR(20)
);
CREATE TYPE MenuType AS (
bar REF BarType,
beer REF BeerType,
price FLOAT
);
 Create Tables of UDTs
 CREATE TABLE Bars OF BarType;
 CREATE TABLE Beers OF BeerType;
 CREATE TABLE Sells OF MenuType;
Example
 Querying:
 SELECT * FROM Bars;
 Produces “tuples” such as:
 BarType(’Joe’’s Bar’, ’Maple St.’)
 Another query:
 SELECT bb.name(), bb.addr()
 FROM Bars bb;
 Inserting tuples:
 SET newBar = BarType();
 newBar.name(’Joe’’s Bar’);
 newBar.addr(’Maple St.’);
 INSERT INTO Bars VALUES(newBar);
Example
 UDT’s can be used as types of attributes in a table
CREATE TYPE AddrType AS (
street CHAR(30),
city CHAR(20),
zip INT
);
CREATE TABLE Drinkers (
name CHAR(30),
addr AddrType,
favBeer BeerType
);
 Find the beers served by Joe:
SELECT ss.beer()->name
FROM Sells ss
WHERE ss.bar()->name = ’Joe’’s Bar’;
An Alternative: OODBMS
 Persistent OO programming
 Imagine declaring a Java object to be “persistent”
 Everything reachable from that object will also be persistent
 You then write plain old Java code, and all changes to the persistent
objects are stored in a database
 When you run the program again, those persistent objects have the
same values they used to have!
 Solves the “impedance mismatch” between programming
languages and query languages
 E.g. converting between Java and SQL types, handling rowsets, etc.
 But this programming style doesn’t support declarative queries
 For this reason (??), OODBMSs haven’t proven popular
 OQL: A declarative language for OODBMSs
 Was only implemented by one vendor in France (Altair)
OODBMS
 Currently a Niche Market
 Engineering, spatial databases, physics etc…
 Main issues:
 Navigational access
 Programs specify go to this object, follow this pointer
 Not declarative
 Though advantageous when you know exactly what you want,
not a good idea in general
 Kinda similar argument as network databases vs relational
databases
Summary, cont.
 ORDBMS offers many new features
 but not clear how to use them!
 schema design techniques not well understood
 No good logical design theory for non-1st-normal-form!
 query processing techniques still in research phase
 a moving target for OR DBA’s!
 OODBMS
 Has its advantages
 Niche market
 Lot of similarities to XML as well…
XML
 Extensible Markup Language
 Derived from SGML (Standard Generalized Markup Language)
 Similar to HTML, but HTML is not extensible
 Extensible == can add new tags etc
 Emerging as the wire format (data interchange format)
XML
<bank-1>
<customer>
<customer-name> Hayes </customer-name>
<customer-street> Main </customer-street>
<customer-city> Harrison </customer-city>
<account>
<account-number> A-102 </account-number>
<branch-name>
Perryridge </branch-name>
<balance>
400 </balance>
</account>
<account>
…
</account>
</customer>
.
.
</bank-1>
Attributes
 Elements can have attributes
<account acct-type = “checking” >
<account-number> A-102 </account-number>
<branch-name> Perryridge </branch-name>
<balance> 400 </balance>
</account>
 Attributes are specified by name=value pairs inside
the starting tag of an element
 An element may have several attributes, but each
attribute name can only occur once
 <account acct-type = “checking” monthly-fee=“5”>
Attributes Vs. Subelements
 Distinction between subelement and attribute
 In the context of documents, attributes are part of markup,
while subelement contents are part of the basic document
contents
 In the context of data representation, the difference is unclear
and may be confusing
 Same information can be represented in two ways
– <account account-number = “A-101”> …. </account>
– <account>
<account-number>A-101</account-number> …
</account>
 Suggestion: use attributes for identifiers of elements, and use
subelements for contents
Namespaces
 XML data has to be exchanged between organizations
 Same tag name may have different meaning in different
organizations, causing confusion on exchanged documents
 Specifying a unique string as an element name avoids confusion
 Better solution: use unique-name:element-name
 Avoid using long unique names all over document by using XML
Namespaces
<bank Xmlns:FB=‘http://www.FirstBank.com’>
…
<FB:branch>
<FB:branchname>Downtown</FB:branchname>
<FB:branchcity>
</FB:branch>
…
</bank>
Brooklyn </FB:branchcity>
Document Type Definition (DTD)
 The type of an XML document can be specified using a DTD
 DTD constraints structure of XML data
 What elements can occur
 What attributes can/must an element have
 What subelements can/must occur inside each element, and how
many times.
 DTD does not constrain data types
 All values represented as strings in XML
 DTD syntax
 <!ELEMENT element (subelements-specification) >
 <!ATTLIST element (attributes) >
Bank DTD
<!DOCTYPE bank [
<!ELEMENT bank ( ( account | customer | depositor)+)>
<!ELEMENT account (account-number branch-name balance)>
<! ELEMENT customer(customer-name customer-street
customer-city)>
<! ELEMENT depositor (customer-name account-number)>
<! ELEMENT account-number (#PCDATA)>
<! ELEMENT branch-name (#PCDATA)>
<! ELEMENT balance(#PCDATA)>
<! ELEMENT customer-name(#PCDATA)>
<! ELEMENT customer-street(#PCDATA)>
<! ELEMENT customer-city(#PCDATA)>
]>
IDs and IDREFs
 An element can have at most one attribute of type ID
 The ID attribute value of each element in an XML document must
be distinct
 Thus the ID attribute value is an object identifier
 An attribute of type IDREF must contain the ID value of an
element in the same document
Bank DTD with Attributes
 Bank DTD with ID and IDREF attribute types.
<!DOCTYPE bank-2[
<!ELEMENT account (branch, balance)>
<!ATTLIST account
account-number ID
# REQUIRED
owners
IDREFS # REQUIRED>
<!ELEMENT customer(customer-name, customer-street,
custome-city)>
<!ATTLIST customer
customer-id
ID
# REQUIRED
accounts
IDREFS # REQUIRED>
… declarations for branch, balance, customer-name,
customer-street and customer-city
]>
XML data with ID and IDREF attributes
<bank-2>
<account account-number=“A-401” owners=“C100 C102”>
<branch-name> Downtown </branch-name>
<balance>
500 </balance>
</account>
<customer customer-id=“C100” accounts=“A-401”>
<customer-name>Joe
</customer-name>
<customer-street> Monroe </customer-street>
<customer-city> Madison</customer-city>
</customer>
<customer customer-id=“C102” accounts=“A-401 A-402”>
<customer-name> Mary </customer-name>
<customer-street> Erin
</customer-street>
<customer-city> Newark </customer-city>
</customer>
</bank-2>
Querying and Transforming XML Data
 Standard XML querying/translation languages
 XPath
 Simple language consisting of path expressions
 Forms a basic component of the next two
 XSLT
 Simple language designed for translation from XML to XML and
XML to HTML
 XQuery
 An XML query language with a rich set of features
Tree Model of XML Data
 Query and transformation languages are based on a tree model
of XML data
bank-2
customer [..]
account
customer [customer-id=“C100”,
accounts=“A-401
branch-name
balance
Downtown
500
XPath
 /bank-2/customer/customer-name
<customer-name>Joe</customer-name>
<customer-name>Mary</customer-name>
 /bank-2/customer/customer-name/text( )
Joe
Mary
 /bank-2/account[balance > 400]
 returns account elements with a balance value greater than
400
 /bank-2/account[balance > 400]/@account-number
 returns the account numbers of those accounts with balance
> 400
Functions in XPath
 /bank-2/account[customer/count() > 2]
 Returns accounts with > 2 customers
 Boolean connectives and and or and function not() can be used
in predicates
 IDREFs can be referenced using function id()
 E.g. /bank-2/account/id(@owner)
 returns all customers referred to from the owners attribute
of account elements.
More XPath Features
 “//” can be used to skip multiple levels of nodes
 E.g. /bank-2//customer-name
 finds any customer-name element anywhere under the /bank-2
element, regardless of the element in which it is contained.
 Wild-cards
 /bank-2/*/customer-name
 Match any element name
XSLT
 A stylesheet stores formatting options for a document, usually
separately from document
 E.g. HTML style sheet may specify font colors and sizes for
headings, etc.
 The XML Stylesheet Language (XSL) was originally designed for
generating HTML from XML
 XSLT is a general-purpose transformation language
 Can translate XML to XML, and XML to HTML
 XSLT transformations are expressed using rules called templates
 Templates combine selection using XPath with construction of
results
XSLT Templates
 Example of XSLT template with match and select part
<xsl:template match=“/bank-2/customer”>
<xsl:value-of select=“customer-name”/>
</xsl:template>
<xsl:template match=“*”/>
 The match attribute of xsl:template specifies a pattern in XPath
 Elements in the XML document matching the pattern are
processed by the actions within the xsl:template element
 xsl:value-of selects (outputs) specified values (here, customername)
 For elements that do not match any template
 Attributes and text contents are output as is
 Templates are recursively applied on subelements
 The <xsl:template match=“*”/> template matches all
elements that do not match any other template
 Used to ensure that their contents do not get output.
Creating XML Output
 Any text or tag in the XSL stylesheet that is not in the xsl
namespace is output as is
 E.g. to wrap results in new XML elements.
<xsl:template match=“/bank-2/customer”>
<customer>
<xsl:value-of select=“customer-name”/>
</customer>
</xsl:template>
<xsl:template match=“*”/>
 Example output:
<customer> Joe </customer>
<customer> Mary </customer>
XQuery
 XQuery is a general purpose query language for XML data
 Currently being standardized by the World Wide Web
Consortium (W3C)
 The textbook description is based on a March 2001 draft of the
standard. The final version may differ, but major features likely to
stay unchanged.
 Alpha version of XQuery engine available free from Microsoft
 XQuery is derived from the Quilt query language, which itself
borrows from SQL, XQL and XML-QL
 XQuery uses a
for … let … where .. result …
syntax
for
 SQL from
where  SQL where
result  SQL select
let allows temporary variables, and has no equivalent in SQL
FLWR Syntax in XQuery
 For clause uses XPath expressions, and variable in for clause
ranges over values in the set returned by XPath
 Simple FLWR expression in XQuery
 find all accounts with balance > 400, with each result enclosed in an
<account-number> .. </account-number> tag
for
$x in /bank-2/account
let
$acctno := $x/@account-number
where $x/balance > 400
return <account-number> $acctno </account-number>
 Let clause not really needed in this query, and selection can be
done In XPath. Query can be written as:
for $x in /bank-2/account[balance>400]
return <account-number> $x/@account-number
</account-number>
Joins
 Joins are specified in a manner very similar to SQL
for $a in /bank/account,
$c in /bank/customer,
$d in /bank/depositor
where $a/account-number = $d/account-number
and $c/customer-name = $d/customer-name
return <cust-acct> $c $a </cust-acct>
 The same query can be expressed with the selections specified
as XPath selections:
for $a in /bank/account
$c in /bank/customer
$d in /bank/depositor[
account-number = $a/account-number and
customer-name = $c/customer-name]
return <cust-acct> $c $a</cust-acct>
XML: Summary
 Becoming the standard for data exchange
 Many details still need to be worked out !!
 Active area of research…
 Especially optimization/implmentation